1. Field of the Invention
The present invention relates to a brewing device for an espresso-type coffee machine, in particular a brewing device in which an amount of ground coffee introduced into a brewing chamber is controlled during each cycle to ensure consistent results for a selected beverage type.
2. Description of the Prior Art
The amount of ground coffee to be introduced into the brewing chamber can be varied to obtain different beverages including a strong espresso known as a ristretto, espresso, a weaker espresso known as a lungo, etc.
Usually, the brewing device of a coffee machine comprises a brewing chamber that is supplied with ground coffee and a pressure piston that compresses the ground coffee in the brewing chamber into a cake. Generally, the ground coffee is produced by a coffee grinder equipped with grinding wheels, one of which is rotated. The amount of ground coffee supplied corresponds to a rotation time of the grinding wheel, this rotation time being stored in a control circuit. However, several parameters can affect the amount of ground coffee supplied for a given rotation time of the grinding wheel, such as the amount of wear of the grinding wheels or the hardness of the coffee beans.
To measure this deviation, it is known to add a measuring device to measure the thickness of the compressed cake of ground coffee, this thickness being representative of the amount of ground coffee introduced into the brewing chamber. The thickness of the cake of ground coffee is calculated based on the measurement of the tamping position of the pressure piston in the brewing chamber.
Thus, document EP0627186 discloses a beverage preparation apparatus that comprises a brewing device comprising a brewing chamber that receives the ground coffee and a pressure piston that is axially displaced by an electric motor from a standby position outside the chamber to a tamping position inside the chamber. A sensor associated with a control device is able to calculate the tamping position of the piston in the chamber by counting the number of revolutions made by the motor. The sensor may be a Hall effect sensor associated with several magnets that are rotated or an optical sensor associated with a slotted disc rotated by the motor. The tamping position of the piston can thus be compared to a predetermined value, a discrepancy between these two values potentially indicating too little or too much ground coffee present in the chamber.
However, in this construction, the sensors and associated moving parts are expensive and sophisticated and can lead to failures. In particular, an optical sensor is susceptible to contamination in the environment surrounding an espresso machine.
Furthermore, document EP0559620 discloses a brewing device comprising a brewing chamber that pivots between a position in which it receives the ground coffee and a tamping position in which the axis of the chamber is aligned with a pressure piston. The pressure piston is axially displaced by an electric motor from a standby position outside the chamber to a tamping position inside the chamber. A sensor associated with a control device is able to calculate the tamping position of the piston in the chamber by counting the number of revolutions made by the motor. The sensor can be an incremental sensor associated with a rotating disc driven by the motor. Alternatively, a linear potentiometer connected to the pressure piston can be used.
While the use of a linear potentiometer appears less expensive than the use of the other sensors mentioned above, it is nevertheless an expensive, bulky component that is difficult to automatically install on an electronic board. The use of a linear potentiometer seems possible only in the case of a brewing device with a short pressure piston stroke, which requires that the displacement of the pressure piston be combined with a movement of the chamber to load the ground coffee. Thus, the mechanics of the brewing device become quite complicated. In addition, since the linear potentiometer has a long stroke, the seal provided by a standard component may be unreliable in the context of a coffee machine.
The object of the present invention is to remedy the drawbacks indicated above and to provide a brewing device that is able to accurately control the amount of ground coffee introduced into a brewing chamber and that is simple in design and inexpensive to implement.
Another object of the present invention is to provide a brewing device that operates reliably.
Yet another object of the invention is to provide an espresso-type coffee machine comprising a brewing device of the type indicated above.